<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }

        .box {
            width: 200px;
            height: 200px;
            background-color: #6ff;
            margin: 50px;
        }
    </style>
</head>

<body>
    <div class="box">
        <?php
        $a = 1;
        $b = 2;
        echo 1 + 2;

        echo "<span>1111111</span>"
        ?>
    </div>
</body>
<script>
    // PHP在服务器上执行，结果以纯文本  HTML json 形式返回给浏览器(前端)  !!!

    var box = document.getElementsByClassName("box")[0];

    box.onclick = function() {
        alert(1111);
    }
</script>

</html>